But because so little data on their earliest members are
available, their evolutionary relationships or phylogeny –
and their place on the tree of life – has remained
unclear.

A new study published Oct. 11 in Royal Society Open Science and
based on improved phylogenetic analysis and advanced computed
tomography (CT) scanning has changed that. The research identifies
two fossils previously thought to be generic carnivorans (a large,
diverse order of mammals) as some of the earliest known members of
the beardog family. These fossils are from animals estimated to be
no larger than about five pounds, roughly the size of a Chihuahua
and much smaller than formidable descendants that would later
evolve.

The work reveals that while evidence of beardogs has been found
throughout the Northern Hemisphere, they may have originated or
initially diversified in parts of what is now the southwestern
U.S.

“Our research pinpoints the southwestern US as a key
region in understanding the diversification and proliferation of
this once successful group of predators prior to their extinction
millions of years ago,” said study coauthor Jack Tseng, PhD,
assistant professor in the Department of Pathology and Anatomical
Sciences in the Jacobs School of Medicine and Biomedical Sciences
at the University at Buffalo.

The evolutionary roots of beardogs

First described back in 1986, fossils found in Texas of animals
believed to be less than 5 pounds were originally assigned to the
genus Miacis, a kind of “miscellaneous” category
for early carnivores, based primarily on external features.

“It was the best that could be done at the time,”
said Tseng, who did the work as a postdoctoral fellow at the
American Museum of Natural History in New York. Susumu Tomiya, PhD,
postdoctoral scholar at The Field Museum in Chicago, is lead author
on the paper.

According to Tseng, the early anatomists based their hypothesis
on superficial features like the shapes of the teeth and the
locations of cranial openings where the blood vessels and nerves
radiate from the brain and other external characteristics,
assigning it to the genus Miacis.

“It’s a kind of ‘trashbin’ genus, when
the question is, well, what else could it be?” Tseng
explained. “Now we’ve taken these fossils out of the
trashbin and put them at the base of the beardog tree.

“We’re not saying we’ve solved where they fit
on the tree of life, but it’s the most progress that’s
been made in quite awhile. Our work provides a clearer connection
between the rest of the beardog family and their evolutionary
roots.”

A mysterious museum specimen

Tomiya decided to study the fossils when he
‘stumbled’ upon one of the specimens in the Field
Museum collection.

“I thought it looked odd and too advanced for what it had
been claimed to be — a more primitive carnivore,” said
Tomiya. “It reminded me of some much larger beardogs so I
decided to take a closer look.”

That closer look included tapping Tseng’s expertise with
high resolution X ray CT 3D reconstructions of the intracranial
anatomy of the fossils. While CT scans of the skull already
existed, Tseng conducted a much more detailed and time-consuming
analysis through more than 1000 slices of CT scans of the skull.
Adding to the difficulty of examining a very small skull —
the whole animal was no larger than a Chihuahua — was the
fact that the spaces inside the skull were still filled with
rock.

Twenty years earlier, Tseng’s PhD advisor, Xiaoming Wang
of the Natural History Museum of Los Angeles County, had speculated
that based on what could be seen externally, this animal was likely
related to beardogs, and may have had a deep embayment — a
bone-enclosed space — in the region of its ear.

“The development of that feature is characteristic of
beardogs,” said Tseng, “and it turns out that that
actually is the case for the skull previously assigned to
Miacis.”

According to Tseng, the ear is very important in understanding
mammalian evolution. “The ear can be used to calibrate how
species are related,” he said, noting that in other
collaborative research, he is studying the ear’s bony
labyrinth, whose shape may be correlated to the kinds of movements
an animal can make.

Tomiya’s work conducting rigorous cladistic analyses
(classification according to shared features) involved updating the
taxonomy of these animals and their evolutionary relationships
based on new phylogenetic affinities— in other words, how the
beardogs may be related to other carnivores that exist today, such
as dogs, bears, raccoons and others.